Porous organic structures, such as a body or film thereof, or structures containing porous organic materials find a variety of industrial uses. Such porous structures are usually mechanically weaker than would be the case if the pores were not present. Moreover, the pores of the structures often intersect the free surfaces thereof, rendering that surface rough, non-uniform or erose. Some potential uses of these porous structures require that the porous structure be rendered mechanically stronger and/or that the free surfaces thereof be smooth and non-erose. One use having these requirements involves the porous structures acting as a dielectric layer in an integrated circuit (“IC”).
Efforts proceed to increase the density of integrated circuits on a single substrate. Circuit density increase requires decreasing the size of integrated circuit features to the low nanometer range. To achieve significant shrinkage in the size of integrated circuit features, new or improved, high-purity, low-K dielectric materials are required to address capacitance-related problems, such as increased power consumption, signal propagation delays and crosstalk between the circuits' conductors and interconnects.
One prospective class of low-K dielectric materials includes porous organic or organic-containing structures into which there have been introduced a high density of nanopores. Typically, these structures comprise thin layers or films containing a large number of small pores or voids that may be open or closed, the latter being preferred. Porous layers or films are deposited or otherwise produced on the substrate for later receipt therein and thereon of the conductors (or lines), pads and interconnects of the integrated circuit supported by the substrate. The pores effect a lowering of the K (dielectric constant or relative permittivity) of the layer or film. Whether the pores of the layer or film are open or closed, some of them typically intersect, and are open at, the free surface of the layer or film.
Metallic conductors of integrated circuits typically begin as metallic patterns on a free surface of the low-K, organic layer or film. The free surface may be a surface of the as-formed layer or a surface exposed after etching of the layer. The metallic patterns are produced by a deposition process such as electroplating a metal layer onto a seed layer of the metal. The seed layer is typically formed by physical vapor deposition (“PVD”) of metal onto a previously deposited barrier layer. The barrier layer is intended to prevent untoward reaction of the atoms or ions of the metal forming the seed layer or the conductor with the material of the organic layer. The free surface may be a generally planar portion of the surface of the layer or film, but more typically comprises the bottom and sidewalls of a conductor-containing trench formed in the organic layer by etching. The metal seed layer and the deposited metal may enter, and contact the walls of pores open at the free surface of the organic layer due to failure of the barrier layer to cover those pore walls. This creates the possibility of unwanted metal-layer reactions. The presence of metal in the pores has also been found to have deleterious effects on integrated circuit operation, particularly in the areas of crosstalk between and among the conductors and the concomitant waste of power. Further, whether or not the barrier layer fully covers the walls of open pores, the seed layer may fail to fully enter some pores. In this event, during the electroplating process, a void in the conductor will occur where no continuous seed layer is present. Such voids may increase the electrical resistance of the conductor to unacceptable levels or, if one or more voids migrate and/or merge, open (render discontinuous) the conductor.
It is desirable that low-K dielectric films in ICs be mechanically strong, especially where metal conductors, having diverse thermal coefficients of expansion, are contained and constrained in trenches. As noted, the presence of pores mechanically weakens dielectric films. Additionally, pores in the dielectric film may have an adverse or unpredictable effect on the thermal expansion coefficient of the film. Pores open at the free surface of the film may increase the moisture uptake of the film and decrease the purity of the film and the adhesion of the film to the underlying surface on which it resides
In view of the foregoing regarding the use of porous dielectric films in IC fabrication, it may be said that, while porosity can impart to a dielectric film a desirable low K, porosity can also have deleterious effects on the quality of the films and or conductors formed therein and thereon.
Supercritical fluids, such as supercritical carbon dioxide (“SCCO2”) are well defined in the art. Supercritical fluids or solutions exist when the temperature and pressure of a solution are above its critical temperature and pressure. In this state, there is no differentiation between the liquid and gas phases and the fluid is referred to as a dense gas in which the saturated vapor and saturated liquid states are identical. Due to their high density, low viscosity and negligible surface tension, supercritical fluids possess superior solvating properties.
Supercritical fluids (“SCF”) have been known for over 200 years. Presently SCFs are used to clean extremely small items, in thin film processing and in other applications as developer reagents or extraction solvents. U.S. Pat. Nos. 5,185,296 and 5,304,515 describe methods in which supercritical fluids are used to remove unwanted organic solvents and impurities from thin films deposited on substrates. U.S. Pat. No. 5,665,527 describes a high resolution lithographic method in which a supercritical fluid is used to selectively dissolve a soluble unexposed portion of a polymeric material (a photoresist) from a substrate, thereby forming a patterned image. In recognition of the superior solvating properties of supercritical fluids, U.S. Pat. No. 5,710,187 describes a method for removing impurities from highly cross-linked porous organic polymers. A key function of the supercritical solvent is to penetrate the nanoporous structure effectively so as to remove all traces of organic solvents and unreacted monomer.
The desirability of sealing or closing, or otherwise eliminating, the open pores intersecting the free surface of the organic layer has been recognized for some time in the IC arena. Effective procedures for achieving this goal without compromising the properties of the film as they relate to integrated circuit fabrication are the subject of continuing work in the field. The present invention presents such an effective procedure.